It's rare for even an elite player to not have a slump now and then, and it's not at all uncommon for slumps to visit other members of the top 20 (or beyond). But what causes an otherwise winning (and uninjured) athlete to suddenly enter a period of poor play--at worst--or--at best--a period of playing well but not being able to close?
There are many reasons.
An obvious one involves returning from an injury layoff. The body may be repaired, but the player has lost her momentum, and may also have a tendency to guard the injured part of her body. Both Elena Dementieva and Maria Sharapova had service problems after they had their shoulders repaired. In fairness, "repaired" shoulders often force players to change their service motions, which creates a whole new learning situation, complete with the anxiety that accompanies such situations.
Another common issue on the WTA Tour is a player's response to sudden success and fame. Petra Kvitova struggled to adjust to being suddenly famous, and the struggle manifested itself in her performance (granted, the Czech isn't the most consistent player, under any circumstance). We don't know, but we might make an educated guess that--to some extent--this kind of pressure also affected Alona Ostapenko. And while we'll never know for sure, we can also speculate about Ana Ivanovic, who just never seemed to quite pull herself together after she won the French Open.
A reason less talked about is mental health vulnerability. Depression, anxiety, ADHD, eating disorders, and posttraumatic stress from trauma all plague athletes, just as they plague everyone else. Having a psychological problem can be devastating, and can cause physical pain, fatigue, increased irritability, an inability to focus, and a loss of interest in one's career.
Closely related is personal stress, caused by such factors as family problems, other relationship problems, or financial problems. In the case of tour athletes, the perception of stress may be magnified because they are traveling and cannot be with their families or significant others (or attorneys), and they may therefore feel powerless and/or excluded.
Also, as we know, the online of abuse of players--primarily, but not entirely--by members of the sports betting community, is a potent source of stress, especially for younger players who lack experience in dealing with really ugly situations.
Then there is the issue of youth, in and of itself. There are some very young players on the tour, and some of them may be even younger in developmental terms, i.e., they may not yet have a level of maturity that matches their age. Given that they probably didn't have "normal" childhoods or adolescent experiences, this isn't a surprise. Some of them may exhibit pseudo-maturity, and are advanced in some ways, but developmentally lagging in others.
Some players take slumps in stride and just go about the business of working their way out of them. A coach should be able to address the slump with the player and help her with problem-solving. But in order to do that, the coach needs to understand what is really going on. That means s/he has to be someone whom the player trusts.
Some players seek the services of sports psychologists or other sports psychology clinicians, which is a good thing, but I wonder whether they may also need to see regular psychotherapists. And that brings up another problem: How can you engage in psychotherapy when you're always traveling? I'm not a fan of tele-psychotherapy, but in this case, it would certainly be better than nothing.
Of course, there's always the possibility that a slump isn't caused by any of these things--that it just appears, like spot of bad weather, and then passes. My guess is that this type of slump is most likely caused by mental fatigue, or a bad match that erodes confidence.